A problem encountered during the treatment of certain hydrocarbon streams of fossil origin is contamination due to the presence of arsenic compounds and sulphur. Besides the resultant toxicological and environmental impacts, these compounds also act as poisons for catalysts of chemical processes, and may affect the catalyst by both physical and chemical adsorption. The presence of these compounds in hydrocarbon streams of fossil origin compromises the performance of catalytic processes for treating them, such as the processes of catalytic hydrotreating (HDT) carried out in refineries. HDT catalysts are vulnerable to these compounds, and may undergo considerable deactivation, promoting operational discontinuity and raising the costs of catalyst replacement.
The feedstocks of the refining processes that are subject to contamination with arsenic compounds include the middle and light distillates derived from materials of fossil origin such as petroleum, schist, bituminous sands or coal, more particularly derived from shale oil, which may possibly form part of feeds of refinery units, whose effluents must undergo the processes of catalytic hydrotreating, putting the catalysts of the process at risk.
Shale oil is an oily stream consisting of organic substances, usually extracted from schist rock by means of retorting processes, basically pyrolysis at temperatures of the order of 450-600° C. The shale oil resulting from these processes may have high contents of arsenic, contaminating compounds of which may appear over the whole range of distillation of shale oil. In particular, streams of distillates in the entire naphtha range (40° C.-290° C.), which are processed in refinery treatment units, such as units for catalytic HDT, may contain arsenic compounds, which act as poison of the catalysts in these units, and put their performance and useful life at risk. Therefore it is necessary to remove said arsenic compounds from these streams.
Gaseous hydrocarbon streams may also contain arsenic compounds, such as trivalent arsines and/or pentavalent arsines. In the case of gas streams produced by hydraulic fracturing of schist in subsoils accompanied by acidification employing acids such as HF or HCl, arsenic compounds in the geological formation may then be lixiviated, generating arsenic acids.
At present, there are various alternatives for removal of arsenic from derivatives of hydrocarbon streams, including shale oil. The commonest processes include processes of removal by coking of schist rock followed by washing with water or with caustic solution, as well as catalytic removal in the presence of pressurized H2, employing guard beds containing spent hydrotreating catalysts based on nickel or cobalt or molybdenum supported on alumina or sulphided silica. The arsenic compounds contained in the gas streams can be removed by pyrolysis processes, where they are collected as metallic arsenic, by adsorption on solids such as zinc oxide or copper oxide, or may also be removed by extraction processes, using an oxidizing solution such as potassium permanganate solution or using organic solvents.
Moreover, processes for catalytic removal are described that employ materials based on oxides or sulphides of iron, nickel or cobalt in the presence of hydrogen at high pressure (of the order of 1500 psig or 10.34 MPa) in guard beds or sludge beds. Arsenic oxides (As2O3) can be removed from gaseous hydrocarbon streams by adsorption on iron oxides at high temperatures (>600° C.) using fixed beds consisting of microparticles of Fe2O3 or CaO or Al2O3. In ambient conditions, iron oxides and hydrated iron oxides have been reported with dearsenization agents of aqueous media, for water purification.
Systems for adsorption of As compounds from aqueous media using iron oxides as adsorbent have been investigated and are well known. Possible materials are Fe2O3, whether hydrated or not, Fe(OH)3, FeOOH, limonite or laterites.
In this connection, the document Wainipee, W, “The effect of crude oil on arsenate adsorption on goethite”, (Univ London Imperial Coll Sci Technol & Med) WATER RESEARCH, 44 (19): 5673-5683 Sp. Iss. SI November 2010, discloses a study on the adsorption of arsenate (As(V)) on the surface of synthetic goethite (α-FeOOH) and of goethite coated with oil in conditions that simulate the conditions of oil field wastewater (aqueous solution containing Na+HAsO4−, at a temperature of 25° C. and with controlled NaCl concentration and pH). This document discloses that, in both cases, adsorption is rapid, but without appreciable removal of arsenic. It was demonstrated that the mechanism of adsorption is described better with the Langmuir model, as the capacity for adsorption increases with decrease in pH, which reflects the increase in positive charges on the surface of goethite. Furthermore, the FTIR results show that As(V) interacts with the carbonyl functional groups of the oil and is removed exclusively in the form of inorganic arsenide ions (HAsO4) present in wastewater from petroleum production, mainly offshore production, which must be treated before being discharged back into the natural environment around the platform.
Moreover, document U.S. Pat. No. 3,876,533 deals with a method for removing impurities or contaminants that poison catalysts, such as arsenic and selenium, from hydrocarbon fluids, such as crude synthetic oil and synthetic oil fractions, by hydrogenation under partial pressure of at least 1500 psi on the surface of particles of Fe2O3, Fe3O4, Ni2O4, Ni3O4, Co2O3, Co3O4 or their respective sulphides, without the need to use aqueous or hydrophilic solutions. This document shows that a stream of shale oil containing about 80 ppm of As is mixed with hydrogen and then is passed through a fixed bed containing particles of Fe2O3 in extruded pellets of cylindrical shape, at a temperature of 371° C. and a pressure of about 100 bar. The residence time is sufficient to allow the contaminant to be removed from the oil and deposited on at least the surface layer of the particles of material, giving 87.5% reduction in total As content.
The document Viet, P H, et al., “Investigation of arsenic removal technologies for drinking water in Vietnam”, ARSENIC EXPOSURE AND HEALTH EFFECTS V (2003) 459-469 deals with a method for reducing the concentrations of As(III) and As(V) in the form of dissociated ions (AsO33− and AsO43−) dissolved in potable water. As shown in the results of sorption experiments, the co-precipitation of arsenate [As(V)] on ferric hydroxide is much more efficient than that of arsenite [As(III)], it being possible to reduce the content of As(V) by more than 90%. In this study, so that they could be used as adsorbent, limonite and laterite were treated by alkaline washing and heating at 900° C., converting crystalline phases of FeOOH to Fe2O3, which has potential as adsorbent of anionic arsenic species.
Finally, document U.S. Pat. No. 6,544,409 deals with a process for the simultaneous removal of sulphur, nitrogen and unsaturated compounds aided by the catalytic action of limonite clays in the presence of a peracid. In this process there is extractive oxidation of unstable sulphurized, nitrogenated and unsaturated compounds by an aqueous solution containing RCOOH, H2O2 and natural goethite (limonite), which acts as a catalyst, permitting the generation of oxidizing free radicals in mild conditions (atmospheric pressure and maximum temperature of approximately 80° C.). The process makes use of the dispersive character of pulverized limonite ore in oil so as to perform direct Fenton-type oxidation of sulphur and nitrogen present in an oil phase, it being especially suitable for the removal of sulphur, nitrogen and unsaturated compounds from light, middle and heavy distillates obtained from petroleum, liquefied coal, shale oil and tar, preferably heavy diesel oil or gas oils from petroleum.
Thus, it can be seen that there are no reports in the prior art that anticipate a process for removing arsenic and sulphur compounds using natural α-FeOOH (goethite), in the absence of hydrogen and at atmospheric pressure.